The present application is the national stage under 35 U.S.C. 371 of PCT/FI98/00558, filed Jun. 25, 1998.
The present invention relates to the art of papermaking. In particular the invention relates to papermaking processes wherein calcium carbonate is included in a pulp suspension, Specifically, the present invention provides a process for preventing or substantially retarding the dissolution of calcium carbonate in the papermaking system. The invention also provides processes for the production of paper or board in papermaking systems wherein solid calcium carbonate is present as a filler and/or pigment.
The pulp suspension used according to the present invention may comprise recycled pulps or virgin mechanical or chemical pulps, or mixtures thereof. Special benefits of the present invention are obtained when calcium carbonate containing recycled fibers are processed into paper in accordance with the invention.
Calcium carbonate CaCO3 is commonly used in paper making as a filler or pigment because it has a high brightness and it is the whitest filler in the price range in question. The calcium carbonate may be naturally occurring chalk or calcite or it may be synthetically produced precipitated calcium carbonate (PCC). Calcium carbonate is sparingly soluble in alkaline conditions above a pH of about 8, but it is attacked by acids such as sulfuric acid and alum, as a result of which it is solubilized. Consequently, normal calcium carbonate is no t a suitable filler for paper making at an acidic pH.
In an attempt to solve the problem with solubilization of calcium carbonate at acidic pH an acid resistant precipitated calcium carbonate has been provided. However, production of this calcium carbonate is technically complicated making the use thereof expensive and even so this product is not either totally acid resistant.
Some papermakers have converted their processes from acidic to neutral pH, partly in order to be able to use calcium carbonate as a filler and/or pigment. The expression xe2x80x9cneutral pHxe2x80x9d corresponds in these processes to a pH in the short circulation of approximately 7-8.5, most preferably 7-8. This applies especially to paper produced from bleached chemical and mechanical pulps. The expression xe2x80x9cpseudoneutral pHxe2x80x9d refers to a pH below that at which calcium carbonate dissolves and it generally refers to a pH of 7 or lower.
In the papermaking processes calcium carbonate is added as a filler to the stock prior to paper formation and consequently a part of the filler particles will enter the process waters circulating in the papermaking system. When calcium carbonate is used as a pigment in coated papers, a part of the calcium carbonate will be recirculated to the process with the broke.
Recycled waste paper as well as broke (herein generally referred to as recycled fibers) may contain calcium carbonate as filler and/or pigment. The repulping of recycled fibers is generally performed at an alkaline pH wherein the calcium carbonate remains essentially in solid form. However, if the paper machine is run at an acidic, neutral or pseudoneutral pH range, the calcium carbonate deriving from the recycled fibers will start to dissolve.
Also under alkaline conditions the stock preparation and the short circulation includes addition of a number of paper chemicals and dilution waters, some of which are acidic and therefore decrease the pH of the pulp. At each acidic addition calcium carbonate may be lost and there may be foaming problems due to a decomposition of dissolved calcium carbonate.
Solubilized calcium carbonate dissociates in water according to the following equations
CaCO3⇄Ca2++CO32xe2x88x92
CO32xe2x88x92+H2O⇄HCO3xe2x88x92+OHxe2x88x92
HCO3xe2x88x92+H2O⇄H2CO3+OHxe2x88x92
H2CO3⇄CO2+H2O
At a pH below about 8 the dissolution of calcium carbonate and consequently the concentration of free calcium ions increases and foaming is observed as carbon dioxide gas is released. With use of closed circulating waters in the papermaking system, the solubilization of calcium carbonate accumulates high concentrations of calcium ions which cause complex problems in the papermaking. Among these problems there may be mentioned coagulation of sticky particles, soap and ink particles; precipitation of inorganic calcium salts as a scaling; precipitation of calcium oxalate and reprecipitation of calcium carbonate; a decrease in the swelling ability of the fibers; interference with retention aids, dispersants and other charged paper additives; etc.
Consequently, there exists a need to improve the use of calcium carbonate in paper making processes.
Carbon dioxide is a gas, which dissolves in water or a pulp suspension forming carbonic acid and/or bicarbonate ions according to the reaction:
xe2x80x83CO2+H2O⇄H2CO3⇄H++HCO3xe2x88x92
Use of carbon dioxide in paper making has been suggested in the prior art for various reasons. According to U.S. Pat. No. 1,993,265 carbon dioxide is used for inhibiting the destructive action of calcium carbonate on a rosin size precipitated with alum.
According to U.S. Pat. No. 2,114,809 a calcium carbonate containing stock is sized using alum, whereby carbon dioxide is created in the reaction between alum and carbonate filler.
According to U.S. Pat. No. 5,378,322 bicarbonate ions required for catalyzing non-acidic sizing with alkylketene dimers may be generated by dissociation of carbon dioxide in the aqueous pulp. If calcium carbonate is added as a filler, the catalytic bicarbonate ions may be produced by a reaction between dissolved carbon dioxide and calcium carbonate. However, this combination of carbon dioxide and calcium carbonate is proposed only for a pH down to 8.6.
According to U.S. Pat. No. 5,262,006 precipitation of gypsum in an alkaline recycle or broke derived pulp may be prevented by adding carbon dioxide to form bicarbonate ions in the alkaline pulp and to precipitate the calcium as calcium carbonate providing PCC in the processing system.
According to EP Patent 0 296 198 the washing of alkaline pulps may be improved by adding carbon dioxide to the washing water.
According to EP Patent 0 281 273 carbon dioxide may be used for adjusting the pH of alkaline pulps upstream of the fibrilation step.
According to GB Patent Application 2 008 562 carbon dioxide may be used for increasing the solubility of calcium carbonate and for the hardening of recycled waters used in the treatment of pulp from waste paper.
It is well known in the art that the solubility of calcium carbonate increases with decreasing pH. According to literature (D. Eklund, T. Lindstrxc3x6m, Paper Chemistryxe2x80x94an introduction, DT Paper Science Publications, Grankulla, Finland 1991, p. 253) an increase in the carbon dioxide partial pressure increases the solubility of calcium carbonate. Consequently, the prior art papermaking processes have recommended the use of calcium carbonate as a filler for alkaline but not for acidic or neutral papermaking.
It has now surprisingly been found that carbon dioxide may be used to improve the function of calcium carbonate in paper making processes operating at pH levels below those at which calcium carbonate traditionally has been used.
Consequently, the present invention relates to a process for reducing or substantially retarding the dissolution and/or dissociation of calcium carbonate in a papermaking system. The process comprises the steps of providing an aqueous pulp suspension in said papermaking system; causing solid calcium carbonate to be present in said pulp suspension; and introducing carbon dioxide to said pulp suspension in connection with subjecting said calcium carbonate in said papermaking system to pH conditions of about 8 or lower.
The carbon dioxide is introduced in an amount sufficient to significantly retard dissolution of said calcium carbonate in said pulp suspension and reduce the amount of free calcium ions in said papermaking system compared to a similar system operating without carbon dioxide.
The fact that carbon dioxide does retard the dissolution of calcium carbonate is in itself surprising and contrary to the beliefs of the prior art. Without wishing to be bound by any theory, the inventors believe that the effect of the carbon dioxide is due to the increased amount of carbonate ions which result from the dissolution of carbon dioxide in the aqueous medium. These carbonate ions affect the balance of the dissociation equation of calcium carbonate in such a way that calcium carbonate has a lower tendency for dissolving and dissociating. Thus, a large part of the calcium carbonate is maintained in solid form and is removed with the paper web. The amount of free calcium ions in the pulp suspension and in the circulating process waters is significantly reduced and there is no accumulation of calcium ions.
The required amount of added carbon dioxide depends on the pH, on the other process conditions as well as on the amount of calcium carbonate present in the pulp suspension. The amount of carbon dioxide added to the pulp suspension may be significant, up to about 5 to 7 kg/ton or even more. Trials have been made with values between 2 and 15 kg/ton with good results. A high amount of carbon dioxide has a clear pH lowering effect on the pulp suspension and a lower pH is known to increase the solubilization of calcium carbonate. However, in the working of the present invention it was surprisingly found that the carbon dioxide retarded the dissolution of calcium carbonate even though the pH was decreased. Thus, decreasing the pH with carbon dioxide does not have the same negative effects on calcium carbonate as decreasing the pH with other acids.
The calcium carbonate in the pulp suspension may derive from calcium carbonate added to the pulp suspension as a filler or it may be provided by calcium carbonate included as a coating pigment in recirculated broke. Alternatively, the calcium carbonate in the pulp suspension may derive from recycled fibers containing significant amounts of calcium carbonate as filler and/or pigment.
The present invention also provides a process for producing paper or board in a papermaking system wherein solid calcium carbonate is present. The process comprises the steps of providing an aqueous pulp suspension in said papermaking system; causing solid calcium carbonate to be present in said pulp suspension; introducing carbon dioxide to said pulp suspension in connection with subjecting said calcium carbonate in said system to pH conditions of about 8 or lower, said carbon dioxide being added into said pulp suspension in an amount which substantially retains said solid calcium carbonate in an undissolved state for a time sufficient for said stock to be processed into a web; feeding said calcium carbonate containing pulp suspension via stock preparation steps to a web forming device; and processing said web into paper having calcium carbonate filler.
The present invention further provides a process for producing paper or board from recycled fibers, which process includes the steps of processing recycled fibers including calcium carbonate at an alkaline pH to provide a calcium carbonate containing alkaline recycled pulp suspension; introducing carbon dioxide to said calcium carbonate containing pulp suspension prior to and/or in connection with said calcium carbonate containing pulp suspension being subjected to pH conditions of about 8 or lower; feeding said pulp suspension to stock preparation in a paper making machine; and producing paper including calcium carbonate filler from said stock.
If the pH in a calcium carbonate containing suspension is lowered with a strong acid such as sulfuric acid to the critical level of pH about 8, the calcium carbonate will start to dissolve. The dissolving is generally the quicker the lower the pH is. In some cases it may be acceptable to lower the pH of the pulp suspension to as low as 7.5 or even 7 with other acids, provided that carbon dioxide is introduced into the suspension fairly quickly after the pH decrease below about 8. It is preferable, however, to provide any pH decrease below about pH 8 with the carbon dioxide itself, in which case the solubilization of calcium carbonate will be retarded according to the invention.